1. Field of the Invention
This invention relates to automatic white balance adjusting devices in image sensing apparatus such as video cameras or electronic still cameras.
2. Description of the Related Art
On the automatic white balance adjusting device for the video camera or the like, a wide variety of proposals are being made at present, including those using the color temperature sensor, or the video signal. Of these, a conventional example of the automatic white balance adjusting device using the video signal is described with reference to FIG. 1.
FIG. 1 is a block diagram roughly illustrating the video signal dependent automatic white balance adjusting device which was reported at the national convention of the Television Society in Japan held in the year 1986. The device includes an image pickup element 1, a luminance signal and color signal generating circuit 2, gain control circuits 3 and 4, a color-difference signal generating circuit 5, an encoder 6, gate circuits 7 and 8, clipping circuits 9 and 10, an R-B signal detecting circuit 11, an averaging circuit 12, a comparison amplifier 13 and a tracking correction circuit 14. The gate circuits 7 and 8 through the tracking correction circuit 14 constitute an automatic white balance adjusting circuit 15. In the device of FIG. 1, a light signal entering the image pickup element 1 is photoelectrically converted to an electrical signal which is applied to the luminance signal and color signal generating circuit 2. In the aforesaid luminance signal and color signal generating circuit 2, a luminance signal Y.sub.H having a luminance signal band, another luminance signal Y.sub.L of color signal band, and a color signal R (Red) and another color signal B (Blue) are generated. The color signal R and the color signal B are applied respectively to the gain control circuits 3 and 4, in which they are amplified by a control signal from the tracking correction circuit 14, and from which they are produced as a color signal R' and a color signal B' respectively. These signals R' and B' are applied along with the aforesaid luminance signal Y.sub.L to the color-difference signal generating circuit 5 in which color-difference signals (R-Y.sub.L) and (B-Y.sub.L) are generated. The color-difference signals (R-Y.sub.L) and (B-Y.sub.L) are applied along with the luminance signal Y.sub.H to the encoder 6. From these inputs, a standard television signal is then generated to be output. Here, the above-described color-difference signals (R-Y.sub.L) and (B-Y.sub.L) are applied also to the automatic white balance adjusting circuit 15.
The color-difference signals (R-Y.sub.L) and (B-Y.sub.L) are applied to the gate circuits 7 and 8 respectively, wherein the unnecessary signal in the blanking period, the abnormal color-difference signal due to the signal collapse at the time of high brightness shooting, etc. are removed. Signals produced from the gate circuits 7 and 8 are applied to the clipping circuits 9 and 10 respectively, wherein any of the color-difference signals which exceed the level for an actually usable color temperature range is clipped, and supplied therefrom to the R-B signal detecting circuit 11, wherein a signal (R-B) is detected by taking difference between the outputs (R-Y.sub.L)' and (B-Y.sub.L)' from the clipping circuits 9 and 10. In the averaging circuit 12, the signal (R-B) from the R-B signal detecting circuit 11 is averaged, thus being converted to a D.C. signal. In the comparison amplifier 13, the signal from the averaging circuit 12 is compared with a reference voltage Vref. A corresponding signal to this comparison is output to the tracking correction circuit 14. In the tracking correction circuit 14, control signals for controlling the gains of the above-described gain control circuits 3 and 4 so as to correct the white balance are generated on the basis of the signal from the comparison amplifier 13. These control signals are output to the above-described gain control circuits 3 and 4. Since a negative-feedback loop is thus formed, the above-described white balance-adjusted color-difference signals can be supplied to the encoder 6.
With such a conventional device as described above, for the light of the sort which approximately comes under Planck's radiation law, such as the sun light or the light from a halogen lamp, when the object to be photographed is white, the values of the signals (R-Y.sub.L) and (B-Y.sub.L) both become zero. Hence, the condition in which the white balance is proper can be established. But, in the case of a light source such as fluorescent lamp, because its intensity distribution over spectrum is different from Planck's radiation law, while the value of the signal (R-B) becomes zero, the values of the signals (R-Y.sub.L) and (B-Y.sub.L) do not become zero, thus giving rise to a problem that the proper white balance cannot be established.